Lubricant composition and speed reducer using the same, and electrically operated power steering device using the same

ABSTRACT

Disclosed are a lubricant composition obtained by adding a buffer particles having an average particle diameter D 1  of 50 μm&lt;D 1 ≦300 μm to a lubricant, a speed reduction gear filled with the lubricant composition, and an electric power steering apparatus having the speed reduction gear incorporated therein, and noise produced in the speed reduction gear can be made lower than ever before irrespective of the magnitude of backlash in a case where a worm and a worm wheel are combined with each other and without complicating the configuration of the speed reduction gear by the buffer action of the buffer particles added to the lubricant composition, thereby making it possible to reduce noise produced in an automobile caused by the electric power steering apparatus at low cost.

PRIOR ART

The present invention relates to a lubricant composition that can besuitably used for a speed reduction gear having a small gear such as aworm and a large gear such as a worm wheel, a speed reduction gearfilled with the lubricant composition, and an electric power steeringapparatus comprising the speed reduction gear.

BACKGROUND ART

Speed reduction gears are used for electric power steering apparatusesfor automobiles. In a column type EPS (Electric Power Steering System),for example, rotation of an electric motor is transmitted from a smallgear such as a worm to a large gear such as a worm wheel in a speedreduction gear to reduce its speed as well as to amplify its output, andis then provided to a column, thereby torque-assisting a steeringoperation.

Suitable backlash is required to engage the small and large gears whichserve as a speed reduction mechanism. At the time of forward/backwardrotation of the gears and in a case where an automobile travels on apunishing road such as a stone pavement so that a reaction force from atire is inputted, for example, however, a teeth striking sound may, insome cases, be produced due to the backlash. When the teeth strikingsound is transmitted as noise to the inside of the automobile, anuncomfortable feeling is given to a driver.

Therefore, so-called by-layer assembly for selecting a combination ofthe gears such that suitable backlash is obtained to assemble a speedreduction gear has been conventionally carried out. In such a method,however, the productivity is significantly low. Another problem is thateven if the by-layer assembly is carried out, there occurs nonuniformityof a steering torque due to offcenter of a shaft of a worm wheel.

The same problems exist in not only speed reduction gear in electricpower steering apparatus but also general speed reduction gear havingthe small and large gears.

In a speed reduction gear in an electric power steering apparatus, forexample, therefore, it has been proposed that backlash is eliminated byproviding urging means such as a spring member for making it possiblefor a worm shaft to be eccentric toward a worm wheel as well as urgingthe worm shaft in the direction of eccentricity (see Japanese UnexaminedPatent Publication No. 2000-43739, for example).

However, the configuration of the above-mentioned speed reduction gearbecomes significantly complicated, resulting in raised manufacturingcost.

DISCLOSURE OF INVENTION

An object of the present invention is to provide a lubricant compositioncapable of making noise produced in a speed reduction gear lower thanever before irrespective of the dimension of backlash in a case wherethe small and large gears are combined with each other and withoutcomplicating the configuration of the speed reduction gear, a speedreduction gear which hardly produces noise by using the lubricantcomposition, and an electric power steering apparatus using the speedreduction gear.

A lubricant composition according to the present invention ischaracterized by comprising a lubricant, and buffer particles having anaverage particle diameter D₁ of 50 μm<D₁≦300 μm.

According to the present invention, the buffer particles dispersed inthe lubricant composition are interposed in an engaged portion of thesmall and large gears to buffer the lubricant composition from acollision between both their gear teeth surfaces to reduce a teethstriking sound, thereby making it possible to reduce noise produced inthe speed reduction gear. Moreover, the noise can be reduced at low costwithout complicating the configuration of the speed reduction gear onlyby merely adding the buffer particles to the lubricant.

It is preferable that the buffer particles are spherical particles.

An advantage of a case where the spherical buffer particles are used,the fluidity of the lubricant composition is improved, thereby making itpossible to prevent a steering torque produced by the electric powersteering apparatus from being excessively raised.

It is preferable that the buffer particles are formed of a bufferingmaterial whose Young's modulus is 0.1 to 10⁴ MPa.

An advantage of a case where the buffer particles formed of thebuffering material whose Young's modulus is within the above-mentionedrange are used, the effect of reducing a teeth striking sound is furtherimproved, and the rise in a steering torque and the production of asliding sound can be reliably prevented.

It is preferable that the buffer particles are formed of a curedmaterial of curable urethane resin.

The buffer particles composed of the cured material of curable urethaneresin can arbitrarily set characteristics such as the Young's modulus byadjusting the degree of hardness of the urethane It is preferable thatthe mixing ratio of the buffer particles is 20 to 300 parts by weightper 100 parts by weight of the lubricant.

An advantage of a case where the mixing ratio of the buffer particles iswithin the above-mentioned range, the effect of reducing a teethstriking sound is further improved, and the rise in a steering torqueand the production of a sliding sound can be reliably prevented.

The lubricant may be semisolid grease, or may be liquid lubricating oil.

When the lubricant is the grease, it is preferable in using thelubricant composition for the speed reduction gear that the consistencyof the lubricant composition having the buffer particles added theretois No. 2 to No. 000 in terms of an NLGI (National Lubricating GreaseInstitute) number.

When the lubricant is the liquid lubricating oil, it is similarlypreferable in using the lubricant composition for the speed reductiongear that the kinetic viscosity thereof is 5 to 200 mm²/s (40° C.).

A speed reduction gear according to the present invention ischaracterized by comprising a small gear and a large gear, and in thatan area including an engaged portion of both the gears is filled withthe above-mentioned lubricant composition, and is preferable in thatnoise such as a teeth striking sound caused by backlash can be reduced.

Furthermore, an electric power steering apparatus according to thepresent invention is characterized in that an output of a motor forsteering assist is transmitted to a steering mechanism by reducing itsspeed through the above-mentioned speed reduction gear, and ispreferable in that noise produced in an automobile can be reduced at lowcost.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic sectional view showing an electric power steeringapparatus according to an embodiment of the present invention.

FIG. 2 is a cross-sectional view taken along a line II-II shown in FIG.1.

FIG. 3A is a graph showing results obtained in measuring therelationship between a steering angle and a torque in examples of thepresent invention, and FIG. 3B is a graph showing the vicinity of astarting point of FIG. 3A in enlarged fashion.

FIG. 4 is a graph showing results obtained in measuring the relationshipbetween the average particle diameter of buffer particles and a teethstriking sound produced by a speed reduction gear in an electric powersteering apparatus in examples of the present invention.

FIG. 5 is a graph showing results obtained in measuring the relationshipbetween the average particle diameter of buffer particles and a slidingsound produced by a speed reduction gear in an electric power steeringapparatus in examples of the present invention.

FIG. 6 is a graph showing results obtained in measuring the relationshipbetween the Young's modulus of a buffering material forming bufferparticles and a teeth striking sound produced by a speed reduction gearin an electric power steering apparatus in examples of the presentinvention.

FIG. 7 is a graph showing results obtained in measuring the relationshipbetween the Young's modulus of a buffering material forming bufferparticles and a sliding sound produced by a speed reduction gear in anelectric power steering apparatus in examples of the present invention.

FIG. 8 is a graph showing results obtained in measuring the relationshipbetween the mixing amount of buffer particles and a teeth striking soundproduced by a speed reduction gear in an electric power steeringapparatus in examples of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention will be described in detail below.

<Lubricant Composition>

A lubricant composition according to the present invention comprises alubricant and buffer particles, as described above.

The average particle diameter D₁ of the buffer particles must satisfy 50μm<D₁≦300 μm.

When the average particle diameter D₁ of the buffer particles is lessthan 50 μm, the effect of absorbing shock in engagement between a smallgear and a large gear to reduce a teeth striking sound has a limitation,so that noise produced in a speed reduction gear cannot be significantlyreduced. When the average particle diameter D₁ exceeds 300 μm, asteering torque produced by an electric power steering apparatus rises,and a sliding sound is produced so that the noise produced in the speedreduction gear is rather increased.

It is preferable that the average particle diameter of the bufferparticles is particularly not less than 100 μm in the above-mentionedrange, considering that the effect of reducing the teeth striking soundis further improved. Further, it is preferable that the average particlediameter of the buffer particles is particularly not more than 200 μm inthe above-mentioned range, considering that the rise in the steeringtorque and the production of the sliding sound are more reliablyprevented.

Various types of shapes such as a spherical shape, a granular shape, aflake shape, and a stick shape can be selected as the shape of thebuffer particles. However, the spherical shape or the granular shape ispreferable, and the spherical shape is particularly preferable,considering the fluidity or the like of the lubricant composition.

It is preferable that Young's modulus of the one used as the bufferingmaterial forming the buffer particles is in a range of 0.1 to 10⁴ MPa.

A buffering material whose Young's modulus is less than 0.1 MPa is toosoft. Therefore, the effect of reducing the noise produced in the speedreduction gear by absorbing shock with the buffering material interposedin the engaged portion of the gears to reduce the teeth striking soundmay not be sufficiently obtained. When the Young's modulus of thebuffering material exceeds 10⁴ MPa, the steering torque produced by theelectric power steering apparatus rises, and the sliding sound isproduced so that the noise produced in the speed reduction gear may berather increased.

It is preferable that the Young's modulus of the buffering materialforming the buffer particles is particularly not less than 0.5 MPa inthe above-mentioned range, considering that the effect of reducing theteeth striking sound is further improved. Further, it is preferable thatthe Young's modulus of the buffering material is particularly not morethan 10² MPa in the above-mentioned range, considering that the rise inthe steering torque and the production of the sliding sound are morereliably prevented.

The other characteristics of the buffer particles are not particularlylimited. However, it is preferable that the tensile strength of thebuffering material forming the buffer particles is 1 to 50 MPa.

It is preferable that the hardness of the buffering material forming thebuffer particles is not less than 10 in terms of Shore D hardness, andis not more than 110 in terms of Rockwell hardness (R scale).

Usable as such buffer particles are any of various ones, composed ofrubber or soft resin, having rubber elasticity.

Examples of the soft resin include polyolefin resin, polyamide resin,polyester resin, polyacetal resin, polyphenylene oxide resin, polyimideresin, fluororesin, and thermoplastic or curable (crosslinking) urethaneresin. Further, it is also possible to use thermoplastic elastomershaving oil resistance such as thermoplastic elastomer olefin,thermoplastic elastomer urethane, thermoplastic elastomer polyester,thermoplastic elastomer polyamide and thermoplastic elastomer fluorine.

Examples of the rubber include ethylene-propylene copolymer (EPM),ethylene-propylene-diene copolymer (EPDM), silicone rubber, and urethanerubber (U).

However, it is preferable that spherical buffer particles formed of acured material of curable urethane resin are used, considering heatresistance, durability, and so on.

Furthermore, the buffer particles formed of the cured material ofcurable urethane resin also have the advantage that the Young's modulus,the tensile strength, and the hardness can be arbitrarily set byadjusting the degree of hardness (the degree of crosslinking) of theurethane resin.

It is preferable that the mixing ratio of the buffer particles is 20 to300 parts by weight per 100 parts by weight of the lubricant.

In a case where the mixing ratio of the buffer particles is less than 20parts by weight, the effect of reducing the noise produced in the speedreduction gear by absorbing shock with the buffer particles interposedin the engaged portion of the gears to reduce the teeth striking soundmay be insufficient. When the mixing ratio of the buffer particlesexceeds 300 parts by weight, the steering torque produced by theelectric power steering apparatus rises, and the sliding sound isproduced so that the noise produced in the speed reduction gear may berather increased.

It is preferable that the mixing ratio of the buffer particles isparticularly not less than 25 parts by weight in the above-mentionedrange per 100 parts by weight of the lubricant, considering that theeffect of reducing the teeth striking sound is further improved. It ispreferable that the mixing ratio of the buffer particles is particularlynot more than 100 parts by weight in the above-mentioned range,considering that the rise in the steering torque and the production ofthe sliding sound are more reliably prevented.

As the lubricant for dispersing the buffer particles, either liquidlubricating oil or semisolid grease may be used.

Preferably used as the lubricating oil is one whose kinetic viscosity is5 to 200 mm²/s (40° C.) and is particularly 20 to 100 mm²/s (40° C.).

Although synthetic hydrocarbon oil (e.g., poly((x-olefin) oil) ispreferable as the lubricating oil, synthetic oil such as silicone oil,fluorine oil, ester oil, and ether oil, and mineral oil can be alsoused. The lubricating oils can be used alone or may be used incombination.

A solid lubricant (molybdenum disulfide, graphite, PTFE(poly(tetrafluoroethylene)), etc.), a phosphorous- or sulfur-basedextreme-pressure additive, an antioxidant such as tributylphenol ormethylphenol, a rust preventive, a metal deactivator, a viscosity indeximprover, an oiliness improver, etc. may be added, as required, to thelubricating oil.

On the other hand, preferably used as the grease is one whoseconsistency is No. 2 to No. 000 and is particularly No. 2 to No. 00 interms of the above-mentioned NLGI number as the lubricant composition towhich the buffer particles are added.

The grease is formed by adding a thickening agent to lubricating baseoil, as in the conventional example.

Although synthetic hydrocarbon oil (e.g., poly(α-olefin) oil) ispreferable as the lubricating base oil, synthetic oil such as siliconeoil, fluorine oil, ester oil, and ether oil, and mineral oil can be alsoused. It is preferable that the kinetic viscosity of the lubricatingbase oil is 5 to 200 mm²/s (40° C.) and is particularly 20 to 100 mm²/s(40 ° C.).

As the thickening agent, various types of thickening agents (soap- ornon-soap-based thickening agents) conventionally known can be used.

Furthermore, a solid lubricant (molybdenum disulfide, graphite, PTFE,etc.), a phosphrous- or sulfur-based extreme-pressure additive, anantioxidant such as tributylphenol or methylphenol, a rust preventive, ametal deactivator, a viscosity index improver, an oiliness improver,etc. may be further added, as required, to the grease.

<Speed Reduction Gear and Electric Power Steering Apparatus>

FIG. 1 is a schematic sectional view of an electric power steeringapparatus according to an embodiment of the present invention. FIG. 2 isa cross-sectional view taken along a line II-II shown in FIG. 1.

Referring to FIG. 1, in the electric power steering apparatus in thisexample, a first steering shaft 2 serving as an input shaft to which asteering wheel 1 is attached and a second steering shaft 3 serving as anoutput shaft connected to a steering mechanism (not shown) such as arack-and-pinion mechanism are coaxially connected to each other througha torsion bar 4.

A housing 5 for supporting the first and second steering shafts 2 and 3is composed of an aluminum alloy, for example, and is attached to anautomobile body (not shown). The housing 5 comprises a sensor housing 6and a gear housing 7 which are fitted to each other. Specifically, thegear housing 7 has a cylindrical shape, and an annular edge 7 a at itsupper end is fitted in an annular step 6 a in the outer periphery at alower end of the sensor housing 6. The gear housing 7 accommodates aworm gear mechanism 8 serving as a speed reduction mechanism, and thesensor housing 6 accommodates a torque sensor 9, a control board 10, andso on. A speed reduction gear 50 is configured by accommodating the wormgear mechanism 8 in the gear housing 7.

The worm gear mechanism 8 comprises a worm wheel 12 which is rotatableintegrally with an intermediate portion in the axial direction of thesecond steering shaft 3 and whose movement in the axial direction isregulated, and a worm shaft 11 (see FIG. 2) which is engaged with theworm wheel 12 and is connected to a rotating shaft 32 in an electricmotor M through a spline joint 33.

The worm wheel 12 comprises an annular core metal 12 a coupled to thesecond steering shaft 3 so as to be integrally rotatable, and asynthetic resin member 12 b surrounding the core metal 12 a and havingteeth formed on its outer peripheral surface. The core metal 12 a isinserted into a metal mold at the time of resin molding of the syntheticresin member 12 b, for example. The core metal 12 a and the syntheticresin member 12 b are coupled to and integrated with each other by resinmolding in a state where the core metal 12 a is inserted into the metalmold.

The second steering shaft 3 is supported so as to be rotatable by firstand second rolling bearings 13 and 14 arranged with the worm wheel 12interposed on the upper and lower sides in the axial directiontherebetween.

An outer ring 15 in the first rolling bearing 13 is fitted in a bearingholding hole 16 provided inside a cylindrical projection 6 b at a lowerend of the sensor housing 6 and is held therein. An upper end surface ofthe outer ring 15 is abutted against an annular step 17 so that theupward movement in the axial direction of the outer ring 15 relative tothe sensor housing 6 is regulated.

On the other hand, an inner ring 18 in the first rolling bearing 13 isfitted in the second steering shaft 3 by way of interference fit. Alower end surface of the inner ring 18 is abutted against an upper endsurface of the core metal 12 a in the worm wheel 12.

An outer ring 19 in the second rolling bearing 14 is fitted in a bearingholding hole 20 in the gear housing 7 and is held therein. A lower endsurface of the outer ring 19 is abutted against an annular step 21 sothat the downward movement in the axial direction of the outer ring 19relative to the gear housing 7 is regulated.

On the other hand, an inner ring 22 in the second rolling bearing 14 isattached to the second steering shaft 3 so as to be integrally rotatableand with the relative movement in the axial direction regulated. Theinner ring 22 is interposed between a step 23 in the second steeringshaft 3 and a nut 24 tightened into a screw of the second steering shaft3.

The torsion bar 4 penetrates the first and second steering shafts 2 and3. An upper end 4 a of the torsion bar 4 is connected to the firststeering shaft 2 so as to be integrally rotatable by a connecting pin25, and a lower end 4 b of the torsion bar 4 is connected to the secondsteering shaft 3 so as to be integrally rotatable by a connecting pin26. A lower end of the second steering shaft 3 is connected to asteering mechanism such as a rack-and-pinion mechanism, as describedabove, through an intermediate shaft (not shown).

The connecting pin 25 connects the first steering shaft 2 with a thirdsteering shaft 27 arranged coaxially with the first steering shaft 2 soas to be integrally rotatable. The third steering shaft 27 penetrates atube 28 composing a steering column.

An upper part of the first steering shaft 2 is supported on the sensorhousing 6 through a third rolling bearing 29 composed of a needle rollerbearing, for example, so as to be rotatable. A reduced diameter portion30 in a lower part of the first steering shaft 2 and a hole 31 in anupper part of the second steering shaft 3 are fitted to each other withpredetermined play provided therebetween in the direction of rotationsuch that the relative rotation between the first and second steeringshafts 2 and 3 is regulated in a predetermined range.

Referring to FIG. 2, the worm shaft 11 is supported so as to berotatable by fourth and fifth rolling bearings 34 and 35 held by thegear housing 7.

Inner rings 36 and 37 in the fourth and fifth rolling bearings 34 and 35are fitted in corresponding constricted portions of the worm shaft 11.Further, outer rings 38 and 39 are respectively held in bearing holdingholes 40 and 41 in the gear housing 7.

The gear housing 7 includes a portion 7 b opposed to a part of aperipheral surface of the worm shaft 11 in the radial direction.

The outer ring 38 in the fourth rolling bearing 34 for supporting oneend 11 a of the worm shaft 11 is abutted against a step 42 in the gearhousing 7 and is positioned therein. On the other hand, the inner ring36 is abutted against a positioning step 43 in the worm shaft 11,thereby regulating the movement thereof toward the other end 11 b.

The inner ring 37 in the fifth rolling bearing 35 for supporting thevicinity of the other end 11 b of the worm shaft 11 (an end on the sideof a joint) is abutted against a positioning step 44 in the worm shaft11, thereby regulating the movement thereof toward the one end 11a.Further, the outer ring 39 is urged toward the fourth rolling bearing 34by a screw member 45 for pre-load adjustment. The screw member 45 isscrewed into a screw hole 46 formed in the gear housing 7, therebyapplying a pre-load to the pair of rolling bearings 34 and 35 as well aspositioning the worm shaft 11 in the axial direction. Reference numeral47 denotes a lock nut which is engaged with the screw member 45 in orderto fasten the screw member 45 after pre-load adjustment.

In the gear housing 7, an area including at least an engaged portion Aof the worm shaft 11 and the worm wheel 12 is filled with the lubricantcomposition having the buffer particles dispersed therein, previouslydescribed. That is, with the lubricant composition, only the engagedportion A may be filled, the engaged portion A and the whole of aperipheral edge of the worm shaft 11 may be filled, or the whole of thegear housing 7 may be filled.

The present invention is not limited to the above-mentioned embodiment.Various modifications can be made within the range of items described inthe claims of the present invention. For example, the configuration ofthe speed reduction gear according to the present invention isapplicable to speed reduction gears for apparatuses other than theelectric power steering apparatus.

EXAMPLES

The present invention will be described in more detail on the basis ofexamples.

Example 1

200 parts by weight of spherical or granular buffer particles having anaverage particle diameter of 120 μm composed of a thermoplasticelastomer urethane was added to 100 parts by weight of grease obtainedby adding a soap-based thickening agent to poly (α-olefin) oil, followedby uniform mixing, thereby preparing grease serving as a lubricantcomposition whose consistency is No. 00 in terms of an NLGI number.

Comparative Example 1

Grease having no buffer particles mixed therewith was used as it is.

The speed reduction gear in the electric power steering apparatus shownin FIGS. 1 and 2 was filled with each of the greases in the example 1and the comparative example 1, to measure a teeth striking sound. In aworm gear mechanism, a worm made of an iron-based metal and a worm wheelmade of polyamide resin were combined with each other. Backlash was setto 1′ and 4′. The results are shown in Table 1. TABLE 1 noise level (dB)backlash 1′ 4′ Example 1 37 40 comparative 49 55 example 1

From Table 1, it has been confirmed that the grease in the example 1 hasthe effect of more significantly reducing a teeth striking sound, whichis 12 dB in a case where backlash is 1′ and is 15 dB in a case wherebacklash is 4′, as compared with the grease in the comparative example1.

The speed reduction gear in the same electric power steering apparatusas the above-mentioned one was filled with each of the greases in theexample 1 and the comparative example 1, to measure the relationshipbetween a steering angle and a torque. Backlash was set to 4′. Theresults are shown in FIG. 3A. Further, the vicinity of a starting point,having a steering angle of O and having no torque, in FIG. 3A isillustrated in enlarged fashion in FIG. 3B. In the drawings, a solidline indicates the results in the example 1, and a broken line indicatesthe results in the comparative example 1.

From the drawings, in the comparative example 1, there is an idlerunning area where the increase in the torque corresponding to theincrease in the steering angle cannot be seen in the vicinity of thestarting point. Therefore, it has been found that backlash exists.

On the other hand, in the example 1, the torque increases incorrespondence with the increase in the steering angle from the startingpoint, which means that there is no idle running area. The reason forthis is that the buffer particles are interposed between gear teeth ofthe worm and the worm wheel. Therefore, it has been found that thisstate substantially corresponds to a state where there is no backlashbetween the gear teeth. From this cause, it has been confirmed thatnoise can be significantly reduced by reducing the teeth striking sound,as described above.

Examination of Particle Diameter of Buffer Particles

Spherical particles composed of a cured material of curable urethaneresin (Young's modulus: 10 MPa) were used as the buffer particles. Theaverage particle diameter thereof was changed in a range of 10 to 400μm.

40 parts by weight of the above-mentioned buffer particles were thenadded to 100 parts by weight of grease obtained by adding a soap-basedthickening agent to poly(α-olefin) oil, followed by uniform mixing,thereby preparing grease serving as a lubricant composition whoseconsistency is No. 00 in terms of an NLGI number. Further, the averageparticle diameter of the grease having no buffer particles mixedtherewith was defined to 0 μm.

The speed reduction gear in the electric power steering apparatus shownin FIGS. 1 and 2 was filled with the grease, to measure a teeth strikingsound (dB) and a sliding sound (dB). In the worm gear mechanism, theworm made of an iron-based metal and the worm wheel made of polyamideresin were combined with each other.

(Teeth Striking Sound)

In measuring the teeth striking sound, backlash (B/L) was set to 2′,3.5′, and 5′.

Letting 55 dB be a threshold value, the effect of reducing the teethstriking sound was estimated to be good in a case where the measuredteeth striking sound was not more than the threshold value, while beingestimated to be bad in a case where the measured teeth striking soundexceeded the threshold value. The results are shown in FIG. 4. Atransverse line (a thick line) in the drawing indicates the thresholdvalue. Further, a line —□—, a line —▴—, and a line —●— respectivelyindicate the results of the measurements in cases where the backlash isset to 2′, 3.5′, and 5′.

From FIG. 4, the good effects of reducing the teeth striking sound wererespectively obtained in cases where the average particle diameter ofthe buffer particles was not less than 50 μm under the conditions of thebacklash 2′, was 80 μm or more under the conditions of the backlash3.5′, and was not less than 100 μm under the conditions of the backlash5′.

This has confirmed that the average particle diameter of the bufferparticles must exceed 50 μm and particularly, is preferably not lessthan 100 μm.

(Sliding Sound)

In measuring the sliding sound, backlash was set to 2′.

Letting 55 dB be a threshold value, the sliding sound was estimated tobe absent (good) in a case where the measured sliding sound was not morethan the threshold value, while being estimated to be present (bad) in acase where the measured sliding sound exceeded the threshold value. Theresults are shown in FIG. 5. A transverse line (a thick line) in thedrawing indicates the threshold value.

From FIG. 5, it has been found that the sliding sound can be preventedfrom being produced when the average particle diameter of the bufferparticles was not more than 300 μm.

This has confirmed that the average particle diameter of the bufferparticles must be not more than 300 μm.

Examination of Young's Modulus of Buffer Particles

Spherical particles having an average particle diameter of 150 μmcomposed of a cured material of curable urethane resin were used as thebuffer particles. The Young's modulus of the cured material was changedin a range of 0.01 to 10⁵ MPa.

40 parts by weight of the above-mentioned buffer particles were thenadded to 100 parts by weight of grease obtained by adding a soap-basedthickening agent to poly(α-olefin) oil, followed by uniform mixing,thereby preparing grease serving as a lubricant composition whoseconsistency is No. 00 in terms of an NLGI number.

The speed reduction gear in the same electric power steering apparatusas the above-mentioned was filled with the grease, to measure a teethstriking sound (dB) and a sliding sound (dB). Both the measurements weremade by setting backlash to 2′. Further, a threshold value to be a basisfor the estimation was the same as the above-mentioned.

The results of the measurement of the teeth striking sound are shown inFIG. 6, and the results of the measurement of the sliding sound areshown in FIG. 7. A transverse line (a thick line) in the drawingsindicates the threshold value.

From FIG. 6, the good effect of reducing the teeth striking sound wasobtained when the Young's modulus of the cured material of curableurethane resin serving as the buffering material forming the bufferparticles was not less than 0.1 MPa.

This has confirmed that the Young's modulus of the buffering materialforming the buffer particles is preferably not less than 0.1 MPa.

From FIG. 7, it has been found that the sliding sound can be preventedfrom being produced when the Young's modulus of the cured material ofcurable urethane resin serving as the buffering material forming thebuffer particles is not more than 10⁴ MPa.

This has confirmed that the Young's modulus of the buffering materialforming the buffer particles is preferably not more than 10⁴ MPa.

Examination of Mixing Ratio of Buffer Particles

Spherical particles having an average particle diameter of 150 μmcomposed of a cured material of curable urethane resin (Young's modulus:10 MPa) were used as the buffer particles.

The above-mentioned buffer particles were then added by changing themixing ratio thereof in a range of 0 to 50 parts by weight per 100 partsby weight of grease obtained by adding a soap-based thickening agent topoly(α-olefin) oil, followed by uniform mixing, thereby preparing greaseserving as a lubricant composition whose consistency is No. 00 in termsof an NLGI number.

The speed reduction gear in the same electric power steering apparatusas the above-mentioned was filled with the grease, to measure a teethstriking sound (dB). The measurement was made by setting backlash to 2′.Further, a threshold value to be a basis for the estimation was theabove-mentioned.

The results of the measurement are shown in FIG. 8. A transverse line (athick line) in the drawing indicates the threshold value.

From FIG. 8, the good effect of reducing the teeth striking sound wasobtained when the mixing amount of the buffer particles was not lessthan 20 parts by weight per 100 parts by weight of the grease.

This has confirmed that the mixing amount of the buffer particles is notless than 20 parts by weight.

1. A lubricant composition characterized by comprising a lubricant, andbuffer particles having an average particle diameter D₁ of 50 μm<D₁≦300μm.
 2. The lubricant composition according to claim 1, wherein thebuffer particles are spherical particles.
 3. The lubricant compositionaccording to claim 1, wherein the buffer particles are formed of abuffering material whose Young's modulus is 0.1 to 10⁴ MPa.
 4. Thelubricant composition according to claim 1, wherein the buffer particlesare formed of a cured material of curable urethane resin.
 5. Thelubricant composition according to claim 1, wherein the mixing ratio ofthe buffer particles is 20 to 300 parts by weight per 100 parts byweight of the lubricant.
 6. The lubricant composition according to claim1, wherein grease is used as the lubricant, and the consistency thereofin a state where the buffer particles are added thereto is No. 2 to No.000 in terms of an NLGI number.
 7. The lubricant composition accordingto claim 1, wherein lubricating oil is used as the lubricant, and thekinetic viscosity thereof is 5 to 200 mm²/S (40° C.).
 8. A speedreduction gear characterized by comprising a small gear and a largegear, and in that an area including an engaged portion of both the gearsis filled with the lubricant composition of claim
 1. 9. An electricpower steering apparatus characterized in that an output of an electricmotor for steering assist is transmitted to a steering mechanism byreducing its speed through the speed reduction gear of claim 8.